When a turbo engine air vehicle is at a relatively low speed and a significant engine power increase is required, the problem of intake airflow boundary layer separation at the engine inlet is presented.
Take for example, an aircraft performing take-off operations. The aircraft is initially stationary on the tarmac. The pilot, desiring maximum engine power output, throttles-up the engines. The velocity of the intake airflow prior to entering the engine inlet is relatively low in comparison to the airflow at the engine face within the engine housing or engine nacelle. The engine may be described as rapidly sucking in airflow in order to meet the high power output requirement. Under these conditions (relative low aircraft speed and high engine output requirements), unless adequately compensated for in the design of the engine inlet, boundary layer separation of the intake airflow at the engine inlet will likely occur. Another circumstance of when this may occur is where an aircraft is loitering at a relative low speed (for example, waiting its turn to land at an airport) and a sudden high power output is required (possibly, in order to perform an evasive maneuver).
Boundary layer separation of the intake airflow at the engine inlet results in a significant negative impact on the net power output of the engine. This is because the intake airflow is initially laminar with the airflow being efficiently sucked into and through the engine. Once boundary layer separation occurs, however, the airflow become vortical, having significant localized pressure and direction variations, resulting in a significant loss of engine face pressure. Engine face pressure directly impacts the engine power output performance. In the context of commercial airliners, for example, a loss of 1% of engine face pressure can result in a 1.2-1.5% loss of engine thrust. Moreover, in a worst case scenario, the vortical flow may result in an engine stall.
Previous efforts to address this engine inlet boundary layer separation problem focused at altering the surface contours of the inlet lip. It is known in the art that by smoothing or rounding the leading edge curvature of the inlet lip the boundary separation phenomenon can be altered. This smoothing or rounding of the inlet lip results in a "thick lip" inlet design.
Within the boundary layer, frictional inlet surface forces act to decrease the momentum of the airflow. Boundary layer separation of the airflow results when the momentum of the localized airflow is insufficient to overcome these frictional forces. The effect of rounding the inlet lip is to locally increase the momentum of the airflow within the boundary layer. Thus, the localized airflow is energized as it is swept around the curved thick inlet lip, thereby avoiding boundary layer separation.
The relative thickness of the inlet lip can be described in terms of the ratio of the circular area defined by the diameter defined by the leading edge of a engine inlet to the minimum circular area defined by the interior surface of the engine inlet (lip contraction ratio). In the context of commercial airliners, the industry standard for this lip contraction ratio is 1.33. "Thick" lip designs, however, increase the maximum diameter requirements of the engine nacelle or engine housing, thereby incurring weight, volume, and high speed aerodynamic/drag penalties.
Other previous efforts have employed variable inlet geometry designs. For example, these designs have employed translating engine cowls, where an aerodynamic thin lip inlet has a forward portion which slides forward (on tracks, for example) revealing a localized smooth or rounded inlet lip. Likewise, inlet designs have employed auxiliary inlets or ports, where a thin lip inlet has flaps or doors which reveals a localized smooth or rounded inlet lip. While these designs may address the boundary layer separation problem, they incur significant penalties in relation to weight, volume, airflow leakage, manufacturing costs and maintenance costs.
Accordingly, there is a need in the art for a turbo engine inlet design which addresses inlet boundary layer separation problem without attendant penalties in relation to aerodynamics, weight, volume, and cost.